Railroads are also taking too long to implement safety improvements Congress ordered under legislation passed seven years ago, lawmakers said at a hearing before the Senate's surface transportation panel.

Sen. Richard Blumenthal, D-Conn., the panel's chairman, said he is "disappointed and disturbed by some of the delays and failures in rulemaking and scrutiny."

"One of the things we're going to do here is impose accountability," he said.

Cynthia Quarterman, head of the Pipeline and Hazardous Material Safety Administration, said her agency is working as fast as possible to draft new standards for tank cars used to transport crude oil. She said it takes time to address the more than 100,000 public comments the agency has received, as well as fulfill other requirements of the federal rulemaking process.

The cars, known as DOT-111s, were involved in explosions and fires following derailments of oil trains near Casselton, N.D., in December and Lac-Megantic, Quebec, just across the U.S. border, in July. Forty-seven people were killed in Lac-Megantic and much of the town center destroyed. The North Dakota accident occurred a half mile from Casselton, causing the evacuation of more than 2,000 people.

U.S. freight railroads transported about 415,000 carloads of crude in 2013, up from just 9,500 in 2008, according to government and industry figures. The increase is overwhelmingly due to the fracking boom in the Bakken region, which is mainly in North Dakota, but also extends into parts of Montana and Canada.

The National Transportation Safety Board, which has urged replacement or retrofitting of the tank cars since 1991, calls the original DOT-111 cars still in use an "unacceptable safety risk."

In 2011, the Association of American Railroads issued updated DOT-111 standards in response to the safety criticism. Tank car makers have been using those voluntary industry standards since, but accident investigators and government regulators say those standards don't go far enough, which is why the government is drafting standards now.

Prentiss Searles, testifying for the American Petroleum Institute, said the oil industry hasn't seen evidence that standards need to be toughened beyond the voluntary industry standards. On the other hand, Edward Hamberger, head of the railroad association, said railroads agree new standards are needed.

"This is shaping up as a regulatory fight here," said Sen. Heidi Heitkamp, D-N.D.

In response to questions from Heitkamp, Searles confirmed that older, less safe DOT-111s aren't being taken out of use. Rather, the overall number of tank cars in use has increased, so that there are now a mix of cars designed with the old standards and cars that meet the new voluntary standards.

Blumenthal asked Joseph Szabo, head of the Federal Railroad Administration, why the agency hasn't required commuter railroads like the Metro-North, which serves the New York metropolitan area, to install cameras inside train cabs and speed limiters that have long been recommended by accident investigators. He noted that several new regulations Congress directed the agency to issue in legislation passed seven years ago have still not been completed.

Congress can demand as many regulations as it wants, but only so many regulations can go through the "pipeline" at one time, Szabo responded.

That same safety law requires freight and commuter railroads to instill new technology aimed at preventing the most deadly types of collisions by Dec. 31, 2015. Freight railroads won't meet that deadline, Hamberger said, blaming the Federal Communications Commission for time-consuming locating requirements for communications towers necessary to make the technology work.

Most commuter railroads also aren't expected to meet the deadline.

The technology, known as positive train control, would likely have prevented 24 accidents investigated by the NTSB since 2004, including a Metro-North derailment on Dec. 1 in Bronx, N.Y., in which four people were killed, safety board member Christopher Hart said.